Fix self-test probe isolation and cleanup

[PracticalSwan]

Summary

• run the selected-only Pro context probe even when the write probe is disabled
• use a bounded existing workspace file, or an empty selected-only context, instead of depending on the temporary write probe
• restore a pre-existing .ai-bridge/codexpro-self-test.md exactly after diagnostics, or remove the transient probe and empty directory when self-test created them
• add smoke coverage for disabled write probing, transient cleanup, and pre-existing probe restoration

Fixes #104
Fixes #105

Verification

• npm run build
• node scripts/smoke.mjs
• git diff --check